Some scientific procedures utilize a thermal beam of atoms and sometimes it is advantageous to have a more slowly moving beam of atoms, and in some cases the atoms from the beam are captured, and in some cases the atoms in the beam are used for other applications (e.g., spectroscopy). A common method of generating a beam of atoms uses an evaporator with an outlet. In some cases, the outlet is a thin-walled pinhole, an array of tubes, one or several cylindrical channels, a slit or array of slits, a microchannel plate, or a combination of these items. Atoms are evaporated and exit the evaporator at high speed through the outlet. Atomic collectors typically require atoms to be at low speed in order to be captured, thus they must be slowed between the evaporator and the collector. Other applications for the beam also require low speeds. The Zeeman slower is a well-known method of slowing a beam of atoms, comprising a laser beam and a magnetic field. The laser beam shines into the beam of atoms opposite their direction of travel and is tuned to resonate with a quantum transition in the atoms. When a photon from the laser is absorbed, the atom loses momentum and slows down. When averaged over a large number of absorption-emission cycles, the process of absorbing and re-emitting photons causes a net slowing of the atoms. Due to the Doppler effect, the resonance frequency for a given atom as measured in the lab frame (i.e., the frame for the laser beam) depends on the velocity for the atom. If a fixed frequency laser is used for the cooling, then as the atoms slow down, the frequency difference between the laser and the atomic resonance increases, which causes the photon scattering rate to decrease. The acceleration experienced by the atom depends on this rate, so as the scattering rate decreases, the acceleration decreases. To avoid this problem, a magnetic field is used. The magnetic field counteracts the changing Doppler shift by modifying the resonance frequency of the atoms (e.g., the Zeeman effect). This approach allows the laser beam to continue slowing the atoms as they themselves slow down. For atomic transitions with linear Zeeman shifts the required magnetic field can be derived from a resonance condition: ν0±(μ/h)B=νL=u/λ where ν0 (νL) is the atomic (laser) frequency, μ is the magnetic moment for the transition, B is the amplitude for the magnetic flux density, λ is the wavelength for the atomic transition, and u is the instantaneous speed for the atom. Ideally, the field is axial (e.g., the field lines are collinear with the direction of travel for the atomic beam). Atoms with transitions that have different dependencies of Zeeman shifts could also be slowed by tailoring the magnetic field appropriately to maintain resonance as the atoms are slowed.
Typically the magnetic field is produced with electromagnets. Electromagnets allow for easy customization of the magnetic field to the desired shape. However, electromagnets take up a great deal of space and require power and cooling systems. Although electromagnets can be housed inside vacuum chambers, this approach is impractical for Zeeman slowers given the heat loads and vacuum outgassing generated by the requisite electromagnets. Hence the windings are usually housed outside the vacuum chamber where the atomic beam travels. This results in a large and power-hungry device, unsuitable for some applications.